Neville Boden

Mechanism of quasi‐one‐dimensional electronic conductivity in discotic liquid crystals

Recently, it has been shown that a new class of quasi‐one‐dimensional conductors can be created by doping discotic liquid crystals with appropriate oxidants. This paper reports the elucidation of the mechanism of conduction in these new materials. In particular, the ac conductivity of 2,3,6,7,10,11‐hexahexyloxytriphenylene (HAT6) doped with the Lewis acid AlCl3, has been measured as a function of frequency (10−3–107 Hz), and temperature in its crystalline solid (K), hexagonal discotic liquid crystal (Dho), and isotropic liquid (I) phases. In all three phases the conductivity is independent of frequency at low frequencies, but shows a power law dependence on frequency [σ(ω)∼ωs, s∼0.7–0.8] at higher frequencies. This behavior is characteristic of charge carrier transport by a hopping mechanism. The conductivity data have been analyzed in terms of the Scher and Lax theory to obtain the parameters describing this process. In macroscopically aligned K and Dho phases, the conductivity measured along the column ...

One-dimensional electronic conductivity in discotic liquid crystals

Abstract The discotic mesogen 2,3,6,7,10,11-hexa-hexyloxytriphenylene (HAT6) forms a columnar hexagonal phase which is an electrical insulator. Doping with 1 mol% of the Lewis acid AlCl 3 converts it to a p-type semiconductor with the preferred direction of conduction being along the axes of the columns. The electrical conductivity is envisaged to arise from the migration of positive holes created in the π-electron band of the triphenylene stack. The behaviour is established by electrical conductivity measurements, which show anisotropy, and ESR lineshapes which are consistent with Dysons theory of resonance absorption by conduction electrons.

Are “nematic” amphiphilic liquid crystalline mesophases thermodynamically stable?

Abstract Microscopy and deuterium NMR spectroscopy are employed to demonstrate the existence of a thermodynamically independent nematic amphiphilic mesophase in mixtures of perfluoro-octanoate and water between 37.0 to 87.0% by weight of 2H2O and 284.0 to 348.3 K at 1 atmosphere pressure. This phase is in equilibrium with an isotropic micellar solution and a smeetic lamellars mesophase to, respectively, high and low temperatures. The lamellar to nematic transition line changes from first to second order at a tricritical point corresponding to 42.5% of 2H2O and 328.0 K, whilst the nematic to isotropic micellar solution transition is first order at all compositions. The measurements suggest the nematic phase is a solution of disc shaped micelles which orient with their unique axis parallel to the direction of an applied magnetic field.

Device applications of charge transport in discotic liquid crystals

We discuss some of the unique structural and electronic properties of discotic liquid crystals, particularly the nature of the ‘band structure’, and of charge injection from metal electrodes. Attempts have been made to use conducting discotic liquid crystals in electroluminescent devices and in gas sensors. The latter are created by spin-coating a thin film of discotic liquid crystals onto an array of interdigitated electrodes. We discuss the temperature dependence of the conductivity of these new systems, analyse the mechanism of charge diffusion and the effect of adsorbed gases.

Engineering of peptide β-sheet nanotapes

A set of principles are outlined for the design of short oligopeptides which will self-assemble in appropriate solvents into long, semi-flexible, polymericβ-sheet nanotapes. Their validity is demonstrated by experimental studies of an 11-residue peptide (DN1) which forms nanotapes in water, and a 24-residue peptide (K24) which forms nanotapes in non-aqueous solvents such as methanol. Circular dichroism (CD) spectroscopy studies of the self-assembly behaviour in very dilute solutions (µm) reveal a simple transition from a random coil-to-β-sheet conformation in the case of DN1, but a more complex situation for K24. Association of DN1 is very weak up to a concentration of 40 µm at which there is a sudden increase in the fraction of peptide in the β-sheet structure, indicative of an apparent ‘critical tape concentration’. This is shown to arise from a two-step self-assembly process: the first step being a transition from a random coil to an extended β-strand conformation, and the second the addition of this β-strand to a growing β-sheet. Both peptides are shown to gel their solvents at concentrations above 2×10 -3 volume fraction: these gels are stable up to the boiling point of the solvents. Rheology measurements on gels of the 24-residue peptide in 2-chloroethanol reveal that the tapes form an entangled network with a mesh size of 10–100 nm for peptide volume fractions 0.03–0.003; the persistence length of the tape is 13 nm or greater, indicative of a moderately rigid polymer; the tapes are about a single molecule in thickness. The mechanical properties of the gels in many respects are comparable to those of natural biopolymers such as gelatin, actin, amylose and agarose.

Self-assembling β-Sheet Tape Forming Peptides

Biological proteins have intrinsically the ability to self-assemble, and this has been implicated in pathological situations called amyloid diseases. Conversely understanding protein self-assembly and how to control it can open up the route to new nanodevices and nanostructured materials for a wide range of applications in medicine, chemical industry and nanotechnology. Biological peptides and proteins have complex chemical structure and conformation. This makes it difficult to decipher the fundamental principles that drive their self-assembling behaviours. Here we review our work on the self-assembly of simple de novo peptides in solution. These peptides are designed so that: (i) the chemical complexity of the primary structure and (ii) the conformational complexity are both kept to a minimum. Each peptide adopts an extended β-strand conformation in solution and these β-strands self-assemble in one dimension to form elongated tapes as well as higher order aggregates with pure antiparallel β-sheet structure, without the presence of any other conformations such as turns, loops, α-helices or random coils. Experimental data of the self-assembling properties are fitted with an appropriate theoretical model to build a quantitative relationship between peptide primary structure and self-assembly. These simple systems provide us with the opportunity to reveal the generic properties of the pure β-sheet structures and expose the underlying physicochemical principles that drive the self-assembling behaviour of this biological motif.

Effects of side-chain length on the charge transport properties of discotic liquid crystals and their implications for the transport mechanism

Four homologous mesogens of the hexakis(n-alkoxy)triphenylene series (HATn, n= 4,6,9,11), have been doped with the one electron oxidant NOBF4, and the electrical conductivity measured as a function of frequency in the hexagonal columnar Dh phase. The limiting low-frequency conductivities both parallel, σ‖, and perpendicular, σ⊥, to the column axes decrease exponentially with increasing side-chain length. The value of σ‖ decreases by three orders of magnitude in going from n= 4 to n= 11. This can be understood in terms of an increasing degree of disorder in the packing of molecules in the columns. The decrease of σ⊥ by some two orders of magnitude can be attributed to the increase in the distance between the columns.

Designing new lyotropic amphiphilic mesogens to optimize the stability of nematic phases

Abstract The factors which govern the stability of lyotropic amphiphilic nematic phases are delineated and then used to design mesogens which give rise to stable NC and ND phases on dissolution in water. The synthesis and phase behaviour of novel discoid amphiphiles, designed to form NC phases, are described.

Temperature-independent hole mobility in discotic liquid crystals

Experimental measurements are presented of the hole mobilities of four conjugated discotic systems, forming columnar liquid crystals, as a function of temperature. The measurements cover the crystalline/glassy phase and mesophase of these materials. It is a remarkable fact that the mobility is almost independent of temperature in the range 30 °C–170 °C. Various explanations of a weak temperature dependence exist and these are explored. They include the small polaron of Holstein in the nonadiabatic limit and the effect of the dynamic disorder present in the system.

THE DESIGN AND SYNTHESIS OF SIMPLE MOLECULAR TETHERS FOR BINDING BIOMEMBRANES TO A GOLD SURFACE

Abstract Molecular tethers have been synthesised for fixing biomembranes to a gold surface. These are comprised of a thiol at one end to bind to the gold, a polyethylenoxy chain of defined length (two, six or twelve ethylenoxy units) in the middle and a cholesteryl residue at the other end to insert into the biomembrane.